The present invention relates generally to overhead valve internal combustion engines, and more particularly to the valve train of such an engine.
Air cooled overhead valve internal combustion engines have a cylinder head in which the intake and exhaust valves are slidingly mounted and oriented such that the valve stems extend from the valve head in a direction generally away from the top of the cylinder bore. Push rods engaging valve lifters actuated by a cam shaft in the cylinder portion of the engine extend into the cylinder head generally alongside the valve stems. Disposed between each valve stem and corresponding push rod is a rocker arm pivotally mounted on a rocker arm stud connected to the cylinder head. The rocker arm engages the push rod at one end and the top of the valve stem at the other end and serves to translate upper movement of the push rod to a corresponding downward movement of the valve stem.
Prior rocker arm studs are usually fastened at their lower end to the cylinder head either by a tight press-fit into a corresponding bore or by a threading received in a correspondingly threaded bore. The upper end of the rocker arm stud is usually threaded and received loosely through an elongate hole in the bottom of the rocker arm. A separate spherical-surfaced pivot washer is often disposed about the upper threaded end of the rocker stud in engagement with a corresponding spherical bearing surface inside the rocker arm. A locking type of nut is threaded onto the upper end of the rocker arm stud above the spherical pivot washer and is used to adjust the valve lash by moving the rocker arm pivot point upwardly or downwardly by turning the locking nut with respect to the fixed rocker arm stud to a selected position.
In some overhead valve engines a push rod guide plate is sandwiched between the cylinder head and an integral enlargement of the rocker arm stud. The push rod guide plate includes guide tabs disposed on either side of the push rod to control lateral displacement of the push rod in a direction perpendicular to the rocking plane of the rocker arm.
One disadvantage of the prior art construction of the rocker arm and rocker arm stud is that a significant amount of space is taken up by the locking nut at the top end of the rocker arm stud. Where the locking nut is recessed between the sidewalls of an open-topped stamped rocker arm, sufficient clearance must be provided between the locking nut and the sidewall for an adjustment wrench. In addition, the height of the rocker arm stud must extend above the spherical pivot washer so that the upper threads of the rocker arm stud can be engaged by the locking nut. These two factors increase the width of the rocker arms and the overall height of the valve mechanism with respect to the head gasket surface.